That's kind of what confuses me.
Various articles I have read state that if William is present but Charles isn't, then William's wife - taking precedence from her husband - outranks Camilla. Now, that doesn't make sense to me. That would imply that William outranks Camilla in his own right but how? Because he is the Queen's grandson in his own right whereas Camilla is "just" a Princess by marriage. That sort of make sense unless you realise that then Camilla would actually have to curtsey (in theory) to Andrew, Edward, Harry, Beatrice, Eugenie, Anne, Alexandra... In other words, all royals who were, as it were, born royals.
The only explanation I have is that William - as the eldest son of the Prince of Wales - holds special precedence when his father is not present. Then again, that contradicts to the article you've linked which states he'd have to bow to Camilla whether Charles is present or not. Besides, that would explain why William wouldn't have to bow to Camilla (if Charles isn't present), but not his wife's lack of curtsey (because Kate is in exactly the same position as Camilla). *
I'm starting to believe there was some sort of an error in the interpretation of the Precedence and, to me, everything is pretty simple:
Camilla is the second woman in the Kingdom after the Queen, and there is no one but the Queen and Prince Philip she has to curtsey to whether her husband is present or not. Unless someone gives logical and/or official reason why that isn't the case, I'm going to go by that rule whatever various news outlets may state.
* To clarify, I know royals don't spend the day bowing/curtseying each other; the discussion is from precedence point of view only and has little to do with what royals would do in real life.